In wellbore operations it is beneficial to know the downhole conditions in the wellbore and the surrounding formations. Some examples of downhole conditions or parameters, without limitation, are pressure, temperature, flow and chemical activity. Monitoring of these parameters is beneficial throughout the life of the wellbore and surrounding formation. For example, it is beneficial to monitor these parameters during drilling operations, during production or injection intervals, formation treatment operations (i.e., acidizing, fracturing) and when the wells are shutin. It is further desired, at times to monitor formation parameters away from the producing and injection wellbores by drilling monitoring wellbores. In the past, these parameters and the data representing the parameters were only available at selected times and in selected locations, for example at the wellhead, or when pressure or temperature logs were run. However, with the advent of fiber optic sensors, these parameters can be obtained and monitored in real time and throughout the life of the wellbore and/or formation.
Wellbores by their nature are very limited in space and wellbore operations require conservative utilization of this space. This space limitation is one of the attractive features of fiber optic sensors. For example a fiber optic sensor may be run along the side of a tubular in the wellbore, it may be run separately (typically carried by a plug) or run in a dedicated tubular or in some applications a U-shaped tubular. Fiber optics sensors may also be run, or pumped with a treating fluid through the perforations in the wellbore into the surrounding formation.
However, there are some shortcomings and drawbacks in the current fiber optic sensor technology, in particular in regard to reservoir monitoring using distributed temperature sensors (“DTS”). In DTS assemblies, it is typically required to turn the fiber through 180 degrees in a very small space. Simply bending the fiber tightly is not a satisfactory option because tight bends can cause high bending losses and high bending stresses which increase the probability of fiber failures over time.
A common method for creating tight bends is referred to as micro- or miniature bending, via etching and/or heating and drawing the optical fiber to reduce its diameter to a few microns. Several drawbacks are associated with these methods, including the requirement of specialized equipment and algorithms to precisely control the tapered transition region between the original fiber diameter and the reduced diameter section; and that the reduced diameter section is extremely fragile. These drawbacks are very significant and limiting in the context of fiber optic installations in wellbores.
Wellbores provide very harsh and challenging conditions. Often wellbores encounter very high temperatures, pressures and equipment vibrations from fluid flow. The wellbore environment is very turbulent or violent. Flowing fluid often carries formation material and/or aggregate. Further, wellbore tools and equipment will be positioned throughout the confined spaces of the wellbore. Thus, the fiber optic cable and the fiber turnarounds must be robust.
It is noted that optic fibers utilized in wellbore applications are typically multi-mode, and thus have a larger core diameter than single-mode fibers. Also, the fibers are typically run as part of a fiber optic cable, thus the size of the cable and the space occupied in the wellbore can be significant. This increased sized, due to limited turnaround radius can eliminate some applications.
Therefore, it is a desire to provide an optical turnaround that addresses drawbacks of other systems. It is a farther desire to provide a method and apparatus for turning an optic fiber substantially 180 degrees in a reduced space relative to current micro bending techniques. It is a still further desire to provide a fiber optic wellbore monitoring system utilizing an improved optical turnaround.